CAPNOGRAPHY 

1. Capnography is the continuous measurement and graphical display of CO₂ concentration (or partial pressure) throughout the respiratory cycle.

  • Numeric value → EtCO₂ (End-tidal CO₂)
  • Graph → Capnogram

2. Basic Physiology 

CO₂ Physiology Flow

CO₂ production → transport → elimination:

  1. Cellular metabolism → CO₂ production
  2. Transport via:
    • Dissolved (5%)
    • Carbamino compounds (5–10%)
    • Bicarbonate (80–90%)
  1. Delivered to lungs via pulmonary circulation
  2. Eliminated by alveolar ventilation

Therefore EtCO₂ depends on:

  • Metabolism
  • Cardiac output
  • Ventilation

3. Types of Capnography

A. Based on Sampling

Mainstream Capnography

Sidestream Capnography

Sensor placed directly in airway (between ETT and circuit)

Gas aspirated via sampling tube to analyzer

Real-time measurement (no delay)

Slight delay due to gas transport

Highly accurate waveform

Slightly less accurate (dilution possible)

No gas removal from circuit

Removes small volume of gas (sampling flow)

No dilution of sample

Sample may be diluted (especially with O₂)

Bulky/heavy sensor → risk of tube dislodgement

Lightweight, no added weight on airway

Expensive

More economical

Affected by secretions directly on sensor

Sampling line may get blocked by secretions

Best for intubated patients (ICU/OT)

Can be used in non-intubated patients (nasal cannula)

Not ideal for transport

Portable and convenient

No need for water traps

Requires water trap/filter

Faster response → better for waveform analysis

Slightly slower response

3. Microstream Capnography

  • Modified sidestream
  • Low flow (~50 mL/min)
  • Used in:
    • Neonates
    • NIV patients


4. Capnogram Phases 

Phase I : Dead Space

  • Inspired gas
  • No CO₂

Phase II : Expiratory Upstroke

  • Mixing of dead space + alveolar gas

Phase III : Alveolar Plateau

  • Pure alveolar gas
  • Slight upward slope (normal)

Phase 0: Inspiration

  • Rapid drop to zero

Alpha (α) Angle

Beta (β) Angle

Location: Between Phase II (expiratory upstroke) and Phase III (alveolar plateau)

Location: Between Phase III (alveolar plateau) and Phase 0 (inspiratory downstroke)

Normal Value: ~100–110°

Normal Value: ~90°

Increase in Angle: Indicates delayed/uneven alveolar emptying

Increase in Angle: Indicates presence of CO₂ during inspiration

Causes of ↑ Angle: Bronchospasm, COPD, airway obstruction

Causes of ↑ Angle: Rebreathing (faulty valve, exhausted soda lime)

5. Normal Values

Parameter

Value

EtCO₂

35–45 mmHg

PaCO₂ – EtCO₂ gradient

2–5 mmHg(Due to dead space)

6. Determinants of EtCO₂ 

↑ EtCO₂ (Hypercapnia / Increased EtCO₂)

↓ EtCO₂ (Hypocapnia / Decreased EtCO₂)

Hypoventilation (↓ RR / ↓ tidal volume)

Hyperventilation (↑ RR / ↑ tidal volume)

Sedative / opioid overdose

Pain, anxiety → hyperventilation

Airway obstruction (asthma, COPD – “shark fin”)

Pulmonary embolism (↑ dead space)

Rebreathing (faulty valve, exhausted soda lime)

Shock / low cardiac output states

Increased metabolic activity (fever, sepsis, shivering, seizures)

Cardiac arrest (very low EtCO₂)

Return of spontaneous circulation (ROSC) → sudden rise

Hypothermia (↓ CO₂ production)

Increased cardiac output

Severe hypotension

CO₂ insufflation (laparoscopy)

Hyperventilation on ventilator

Malignant hyperthermia

Neuromuscular paralysis (↓ metabolism)

Thyrotoxicosis

Circuit disconnection / leak

Bicarbonate administration (CO₂ generation)

Esophageal intubation (near-zero EtCO₂)

7. Capnogram Patterns 

Pattern / Shape

Waveform Description

Common Causes

Normal Capnogram

Square waveform with clear phases I–III and sharp downstroke

Normal patient

Bronchospasm (“Shark Fin”)

Slanted expiratory upstroke + prolonged plateau

Asthma, COPD, airway obstruction

Rebreathing

Elevated baseline (does not return to zero)

Faulty valve, exhausted soda lime, low FGF

Curare Cleft

Dip/notch in plateau (Phase III)

Inadequate neuromuscular blockade

Esophageal Intubation

Flat or near-zero waveform after few breaths

Misplaced ETT in esophagus

Sudden Drop in EtCO₂

Abrupt fall in waveform height

Cardiac arrest, shock, Pulmonary embolism, disconnection

Gradual Rise in EtCO₂

Progressive increase in waveform height

Sedation, rebreathing, fever

Cardiac Oscillations

Small rhythmic oscillations on plateau

Seen in low RR or pediatric patients

Leaking Circuit

Irregular waveform with reduced amplitude

Circuit leak, cuff leak

Inspiratory Valve Malfunction

CO₂ present during inspiration (baseline elevated)

Faulty inspiratory valve

Expiratory Valve Malfunction

Prolonged plateau, incomplete emptying

Expiratory valve defect

8. Clinical Uses in CCM 

A. Airway & Ventilation

  • Confirm ETT placement (gold standard)
  • Detect:
    • Accidental extubation
    • Circuit disconnection

B. CPR Monitoring

  • EtCO₂ reflects cardiac output during CPR
  • <10 mmHg → poor compressions
  • Sudden rise → ROSC

9. PaCO₂ vs EtCO₂ Gradient 

Normal: 2–5 mmHg

Increased gradient seen in:

  • ARDS
  • Pulmonary embolism
  • Shock
  • COPD

 Due to increased dead space


10. Capnography in Special Situations

ARDS

  • ↑ dead space → ↑ gradient

Septic Shock

  • Early: ↑ EtCO₂ (hypermetabolic)
  • Late: ↓ EtCO₂ (low CO)

Cardiac Arrest

  • Very low EtCO₂
  • Rise indicates ROSC

11. Limitations

  • Does NOT directly measure PaCO₂
  • Affected by:
    • Dead space
    • Sampling errors
    • Secretions
  • Less reliable in:
    • Severe lung disease
    • Low perfusion states